The present invention relates to radiocommunication systems and, more particularly, to techniques for measuring signal strengths of radio channels.
FIG. 1 depicts a conventional mobile radiocommunication system. Referring to FIG. 1, the mobile radio communication system includes a land system and many mobile stations M1-M10. The land system consists of at least one mobile switching center MSC and several radio base stations B1-B10 distributed over cells C1-C10. Each radio base station serves a cell by sending and receiving information to and from the mobile stations M1-M10 over radio channels. Control channels are used to communicate control and overhead information between the base stations and mobile stations. Traffic channels are each used to support a temporary, dedicated connection (e.g., to handle a voice call) between a base station and a mobile station. The mobile switching center MSC, connected on one side to the public switching telephone network (not shown) and on the other to the base stations B1-B10, performs most of the control functions required for calls to and from the mobile stations M1-M10.
Because the cells C1-C10 are relatively small, the mobile stations M1-M10 often travel between a number of different cells. When a mobile station travels from one cell to another cell, the mobile station switches from a radio channel from a home base station in the cell in which the mobile station originates to a radio channel from a target base station in the cell to which the mobile station travels. The process of switching an established call from a radio channel provided by a base station in one cell to a radio channel provided by a base station in another cell is known as intercell channel reselection.
It is desirable that a mobile station with an established connection on a radio channel to a base station be able to maintain the connection when moving within the same cell, even if the radio channel being used is subject to increased interference. Sometimes interference can be alleviated by changing to a new radio channel within the cell. This process is usually referred to as an intracell channel reselection.
In general, radiocommunication is only possible when the desired information carrying radio signals have sufficiently strong signal strength in relation to noise and interfering radio signals at the receiver in the mobile station. The minimum signal strength of course depends on the particular features of the system, e.g., the kind of modulation and the receiver used. In order to determine if an established connection should continue on a radio channel between a mobile station and a base station or whether intercell or intracell channel reselection needs to be performed, various measurements are made on radio signals received at the mobile station.
Referring again to FIG. 1, the mobile station determines whether intercell or intracell channel reselection is necessary based on signal strengths of the radio channels as measured by the mobile station. In response to a neighbor list order from the land system, the mobile station measures the signal strengths and, possibly, other parameters (e.g., bit error rate) of the radio channel across which it is connected to a home base station and neighboring radio channels emitted by the home base station and/or by the base stations in neighboring cells. Based on the signal strength measurements, the mobile station determines whether reselection to a neighboring radio channel in the same cell or in a different cell is necessary.
Since the radio channel signal strengths measured by the mobile station often contain noise, it is inappropriate to use a single, latest measurement value as the only basis for deciding whether reselection is necessary. Thus, techniques such as averaging are applied to the measured signal strengths, and the mobile station uses the average received signal strength of the radio channels to determine whether reselection is necessary.
Various different types of radio channels are used in radiocommunication systems and need to be measured. For example, for systems employing analog radio channels, the traffic channels and the control channels are typically provided on separate frequencies. Thus, it is a relatively simple matter to measure the signal strength of any particular channel, whether a control channel or a traffic channel, simply by tuning to the designated frequency and taking one or more measurements.
Some systems employ digital radio channels. For example, in a TDMA cellular radiotelephone system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation and data. The time slots are grouped into successive TDMA frames having a predetermined duration. A radio channel thus comprises one or more time slots in each successive TDMA frame. Thus, several radio channels may occupy a single frequency using the TDMA methodology.
In some cases, TDMA traffic channels and control channels have not been intermingled on the same frequencies. For example, the systems specified by interim standard IS-54-B indicated that the control channels should be placed on 21 predetermined control channel frequencies, which 21 frequencies are distinct from those used to support TDMA traffic channels. This scheme allowed, among other things, for the control channels to be readily located by mobile stations accessing a radiocommunication system.
More recently, however, systems have been specified wherein the control channels and the traffic channels can be mixed together on the same frequency. For example, those systems specified by the interim standard IS-136 promulgated by EIA/TIA provide techniques for locating digital control channels that are mixed in with digital traffic channels, i.e., allowing for digital control channels and digital traffic channels to be placed on the same frequency. This creates certain difficulties with respect to the measurement of the channel strength of particular channels.
Consider, for example, a frame containing Digital Control Channel (DCCH) data in some time slots and Digital Voice Channel or Digital Traffic Channel (DTC) data in other time slots. The DCCH slots contain fixed patterns of data, e.g., mostly 1's or mostly 0's. In contrast, the DTC slots contain variable patterns of 1's and 0's, resembling almost pseudo-random patterns. When modulation occurs across the DTC slots, the 1's and 0's are averaged out, producing a flat modulation. However, modulation across the DCCH slots results in a modulation which is not flat, but rather a modulation of mostly 1's or mostly 0's. For this reason, signal strength measurement made in a DCCH slot may be higher than a measurement made in a DTC slot.
FIG. 2 illustrates this concept. Therein, a repeating time slot layout and measured signal strengths over two TDMA frames are depicted. In the slot layout shown in FIG. 2, each frame is divided into six slots. DCCH data occupies time slots 1 and 4, and DTC data occupies time slots 2 and 5 and time slots 3 and 6. As shown in FIG. 2, the signal strength is measured from 2-4 dB higher in the DCCH slots than in the DTC slots. This disparity in measured signal strengths between different time slots on the same frequency means that the conventional technique of simply tuning to a particular frequency on which a particular channel is being transmitted and taking a measurement in an arbitrary time slot may provide an inaccurate result. That is, in order to guarantee an accurate signal strength measurement in a system wherein control channels and traffic channels can be placed on the same frequency, it becomes necessary to identify and measure signal strength in an appropriate timeslot.
This solution, however, is problematic due to the fact that the transmissions on different frequencies may be unsynchronized. Unsynchronized transmissions are illustrated in FIGS. 3a-3c which depict time slots and measured signal strengths over two TDMA frames for different frequencies represented by different channel numbers. Referring to FIG. 3a, the time slots associated with radio channel number 300 on a first frequency are transmitting time slot 2 at time t.sub.0, which may, for example, be a DTC slot. Referring to FIG. 3b, radio channel number 312 on a second frequency is carrying time slot 1 at time t.sub.0, which may, for example, be a DCCH slot. Referring to FIG. 3c, radio channel number 333 on a third frequency is carrying time slot 3 at time t.sub.0, which may, for example, be a DTC slot. As can be seen from FIGS. 3a-3c, signal strength values of radio channel numbers 300 and 333 measured at the beginning of a frame period may be 2-4 dB lower than a signal strength value of radio channel number 312 measured at the same moment. Depending on the timing for neighboring radio channels and the point during the frame period at which the channels are measured, the signal strength for a particular radio channel may vary considerably, for example, as much as 4 dB. Thus, to even attempt to precisely measure the signal strength of a particular TDMA channel, i.e., by measuring in one or more of the specific time slots which comprise that channel, would require re-synchronizing to the TDMA frame structure being transmitted on each new frequency to be measured.
Despite these complexities, it is still important to accurately measure the signal strengths of neighboring radio channels because inaccurate measurements may adversely affect reselection. Some systems have been designed to reselect radio channels in response to a relatively small change in signal strength measurement of, for example, 4 dB. If signal strength measurements can vary as much as 4 dB depending on the time slot of a frame that a signal strength measurement is performed in, the mobile station may be connected to new radio channels more often than necessary or oscillate between two radio channels of equal signal strength. Every time radio channels are changed, the mobile station has to keep its receiver on longer which causes the mobile station to draw more power. Also, because the mobile station is occupied with reselection, there is a chance that data will be missed.
Moreover, due to the lack of synchronization between base station transmissions in some systems, and to avoid additional complexity, it is desirable to measure signal strength without considering in which time slot a measurement is taken. Thus, there is a need for a technique for measuring signal strengths of neighbor list radio channels accurately, without taking into account which time slots of a frame the signal strength measurements are made in.